Hydrogen is supplied to customers connected to a hydrogen pipeline system. Typically, the hydrogen is manufactured by steam methane reforming in which a hydrocarbon and steam are reacted at high temperature in order to produce a synthesis gas containing hydrogen and carbon monoxide. Hydrogen is separated from the synthesis gas to produce a hydrogen product stream that is introduced into the pipeline system for distribution to customers that are connected to the pipeline system. Alternatively, hydrogen produced from the partial oxidation of a hydrocarbon can be recovered from a hydrogen rich stream. Typically, hydrogen is supplied to customers under agreements that require availability and on stream times for the steam methane reformer or hydrogen recovery plant. When a steam methane reformer is taken off-line for unplanned or extended maintenance, the result could be a violation of such agreements. Additionally, there are instances in which customer demand can exceed hydrogen production capacity of existing plants. Having a storage facility to supply back-up hydrogen to the pipeline supply is therefore desirable in connection with hydrogen pipeline operations. Considering that hydrogen production plants on average have production capacities that are roughly 50 million standard cubic feet per day, a storage facility for hydrogen that would allow a plant to be taken off-line, to be effective, would need to have storage capacity in the order of 1 billion standard cubic feet or greater.
The large storage capacity can be met by means of salt caverns to store the hydrogen underground. Hydrogen as well as other gases have been stored in salt caverns. Salt caverns are large underground voids that are formed by adding fresh water to the underground salt, thus creating brine, which is often referred to as solution mining. Caverns are common in the gulf states of the United States where demand for hydrogen is particularly high. Such hydrogen storage has taken place where there are no purity requirements or less stringent (<96% pure) requirements placed upon the hydrogen product. In such case, the stored hydrogen from the salt cavern is simply removed from the salt cavern without further processing. Hydrogen storage has also occurred where there are more stringent purity requirements for the hydrogen within the pipeline, and, therefore, for the hydrogen that has been previously stored in the salt cavern that is to be introduced into the pipeline. In order to comply with the more stringent purity requirements, the stored hydrogen that is removed from the pipeline needs to be further processed to remove contaminants that have been imparted into the stored hydrogen by virtue of its storage within the salt cavern. For instance, U.S. Pat. No. 7,078,011 discloses a temperature swing adsorption unit for removing carbon dioxide and water from a hydrogen stream that has been stored in a salt cavern to produce a hydrogen product stream having the impurity level of the carbon dioxide and water vapor at or below the product purity specification. Such a purified hydrogen product stream can then be reintroduced into the pipeline. However, the implementation of purification equipment can substantially increase the cost and complexity of operating a hydrogen storage cavern.
As will be discussed, among other advantages of the present invention, a method and system for processing hydrogen that has been stored in a salt cavern is disclosed in which the hydrogen to be reintroduced into the pipeline will assist in meeting customer demand in a manner that is more cost effective than methods disclosed in the prior art.